Linear image sensor in CMOS technology

ABSTRACT

A time-delay-integration image sensor comprises a matrix of pixels organized in rows and columns. Each pixel comprises a first photosensitive element, a storage node and a first transfer element connected between the first photosensitive element and the storage node, Each pixel further comprises a second photosensitive element, a second transfer element connected between the second photosensitive element and the storage node, and a third transfer element connected between the storage node and the second photosensitive element of an adjacent pixel of the column. A control circuit is configured to simultaneously command the first and second transfer elements to on state and the third transfer element to off state, and, in a distinct phase, to simultaneously command the first and third transfer elements to on state and the second transfer element to off state.

BACKGROUND OF THE INVENTION

The invention relates to a linear image sensor designed to capture animage by scanning, and in particular to a Time-Delay-Integration (TDI)sensor.

STATE OF THE ART

The principles of a TDI image sensor are described for example in thearticle entitled “A Large Area TDI Image Sensor for Low Light LevelImaging” by Michael G. Farrier et al—IEEE Journal of Solid-StateCircuits, Vol. SC-15, No. Aug.4 , 1980.

A TDI sensor is generally used to capture the image of an object movingat high speed and observed under poor lighting conditions. It isgenerally implemented using CCD (Charge-Coupled Device) technology whichhas so far enabled the best performances to be obtained in terms ofsensitivity.

FIG. 1 schematically represents a TDI sensor in CCD technology asdescribed in the above-mentioned article. It comprises a matrix ofphotosensitive sites, or photosites 10 whereof the rows are generally,as represented, considerably longer than the columns. In the example ofthe above-mentioned article, a row comprises 1028 photosites, whereas acolumn only comprises 128 photosites. For earth photography viasatellite, a row can comprise some 12,000 photosites and the matrixcomprises several tens of rows.

The rows of the matrix are arranged perpendicularly to the motion of theobject whereof the image is to be captured. Motion of this imagerelative to the sensor is represented by descending arrows. These arrowsalso correspond to movement of the electric charges in the CCDregisters, in synchronism with the motion of the image.

Each row captures a corresponding slice of the object during an exposuretime compatible with the speed of the image. This results in anaccumulation of negative charges (electrons) in the photosites of therow.

When a slice of the image captured by a row i is moved to the level ofrow i+1, the charges accumulated in row i are transferred to row i+1,which, during a new exposure time, continues to accumulate charges forthe same slice. Charge transfers from one row to the next therefore takeplace in synchronism with motion of the image.

At each transfer cycle, the last row of the matrix thus contains the sumof the charges accumulated by all the rows for one and the same slice.The sensitivity of the sensor is therefore in theory multiplied by thenumber of rows.

At the end of each charge transfer and exposure cycle, the charges ofthe last row of the matrix are transferred into a shift register 12whereof the purpose is to read the data of the last row. The chargesstored in the photosites of this to register are shifted photosite byphotosite to a charge-voltage converter 14 at the end of the row where avoltage corresponding to the total charge of each photosite can becollected by a processing circuit, generally external to the sensor.

As the CCD technology is less and less used for image sensors to theprofit of the CMOS technology, the use of the latter technology isenvisaged for TDI sensors.

The article entitled “Time-Delay-Integration Architectures in CMOS ImageSensors” by Gerald Lepage, Jan Bogaerts and Guy Meynants—IEEETransactions on Electron Devices, Vol. 56, N° Nov.11, 2009, describessolutions for obtaining the TDI functionality by means of a CMOS imagesensor.

In a CMOS image sensor, light is also captured in the form of charges atpixel level. However, as each pixel is provided with its own voltageread circuit, charges cannot be transferred from one pixel to another.

FIG. 2 schematically represents an architecture envisaged in thisarticle by Lepage et al. A matrix 10′ of N×M pixels Px is associatedwith a matrix 16 of memory cells of the same size and configuration(here N×M=5×5).

In principle, pixel matrix 10′ takes views at a rate corresponding tothe time (called “line time” T_(L)) taken by an image slice to scan thepitch of the rows of pixels. Thus, after N line times, the same imageslice will have been captured by each of the N rows of the pixel matrix.Each row of memory 16 is temporarily associated with the same slice ofthe image. The brightness levels (i.e signal levels) recorded for thisslice by all the rows of pixels is accumulated therein.

Once the levels have been accumulated for the slice, the memory row isread, reset, and associated in circular manner with a new image slice.

It is thus observed that accumulation of all the rows of the pixelmatrix has to be performed at each line time.

Whereas in CCD technology the brightness level accumulation operationscorrespond to simple charge transfers, these operations are notably morecomplex in CMOS technology. They involve multiplexing operations onpixel read busses, analog-to-digital conversions, addition operations,and memory access operations. This results in difficulties in CMOStechnology to achieve the same view capture rates (or line time I_(L))as in CCD technology. The resolution of the pixel matrix in number ofrows therefore has to be adjusted to the minimum line time envisaged andto the desired pixel pitch.

In certain applications, it is sought to improve what is referred to asthe image motion Modulation Transfer Function (MTF) which is one of theparameters representative of the sharpness of the reproduced image. Alow motion MTF generally results in a blurry image. This loss ofresolution is due to the fact that, during a line time, the image slicemoves over the row of pixels which is stationary.

As described in the above-mentioned article by Lepage et al., onesolution consists in subdividing each pixel in two in the direction ofmotion. This involves increasing the number of rows of pixels while atthe same time reducing the pitch to preserve the dimensions of thesensor. The motion MTF progresses from 0.64 to 0.9 when the pixel issubdivided into two equal parts. On the other hand, the temporalconstraints increase with the square of the subdivision factor. They arethus multiplied by 4.

SUMMARY OF THE INVENTION

A need is observed to provide a CMOS image sensor with TDIfunctionnality enabling the image motion MTF to be improved withouthowever increasing the temporal constraints.

This need tends to be satisfied by providing a time-delay-integrationimage sensor comprising a matrix of pixels organized in rows andcolumns, each pixel comprising a first photosensitive element, a storagenode and a first transfer element connected between the firstphotosensitive element and the storage node. Each pixel furthercomprises a second photosensitive element, a second transfer elementconnected between the second photosensitive element and the storage nodeand a third transfer element connected between the storage node and thesecond photosensitive element of an adjacent pixel of the column. Theimage sensor comprises a control circuit configured to simultaneouslycommand the first and second transfer elements to on state and the thirdtransfer element to off state, and, in a distinct phase, tosimultaneously command the first and third transfer elements to on stateand the second transfer element to off state.

A method for management of a time-delay-integration image sensor is alsoprovided.

For each pixel of a column, the method successively comprises the stepsof exposure of the first and second photosensitive elements of the pixelduring a first half period, transfer of the brightness level of thefirst photosensitive element to a storage node of the pixel and transferof the brightness level of the second element photosensitive in thestorage node of the pixel, exposure of the first and secondphotosensitive elements of the pixel during a second half period,transfer of the brightness level of the first photosensitive element tothe storage node of the pixel and transfer of the brightness level ofthe second photosensitive element to the storage node of an adjacentpixel of the column, and read of the brightness levels accumulated inthe storage node of the pixel.

BRIEF DESCRIPTION OF THE DRAWINGS

Other advantages and features will become more clearly apparent from thefollowing description of particular embodiments of the invention givenfor non-restrictive example purposes only and illustrated by means ofthe appended drawings, in which:

FIG. 1, described in the above, schematically represents a conventionalTDI image sensor in CCD technology;

FIG. 2, described in the above, schematically represents a conventionalTDI sensor in CMOS technology;

FIG. 3 represents a four-transistor pixel of a CMOS sensor;

FIG. 4 schematically represents a column of a CMOS TDI sensor enablingthe image motion MTF to be improved;

FIGS. 5 to 8 represent operating steps of the pixels of FIG. 4; and

FIGS. 9A to 9H are timing diagrams illustrating a global operation ofthe pixels of the sensor of FIG. 4.

DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION

FIG. 3 represents a conventional CMOS pixel called “4T” in solid lines.This type of pixel will be thereafter adapted to reduce the motioneffect in a TD1 sensor. It comprises a photodiode D1 which has anintrinsic capacitor C1, or integration capacitor, that enables thecharges generated by the light striking the pixel to be accumulated. Atransfer transistor TG connects photodiode D1 to the gate of a followertransistor M2. The gate capacitor of transistor M2 and the capacitors ofthe other components connected to one and the same node A form a buffercapacitor C2. A selection transistor RS connects the source of followertransistor M2 to a column bus BC. A reset transistor RST connectscapacitor C2 to a positive power supply line Vdd. For reasons ofconvenience, the control signals of the transistors have the same nameas the transistors in the following.

Operation of this pixel is briefly as follows. Capacitor C1 integratesthe charges generated by the light striking photodiode D1. Before theend of exposure, transistor RST is briefly activated to reset buffercapacitor C2. At the end of exposure, transistor TG is briefly activatedto transfer the charges from capacitor C1 to buffer capacitor C2. Ifphotodiode D1 is a pinned photodiode, transfer of charges is total, alsoresulting in capacitor C1 being reset by activation of transistor TG.

Thus, during each exposure phase, the voltage level corresponding to theprevious exposure is stored in buffer capacitor C2. This voltage level,representative of a brightness level, can be transferred at any time tobus BC by activating selection transistor RS, before reset by transistorRST.

In a pixel matrix according to FIG. 3, the pixels of a column share thesame bus BC. The pixels of a column are read row after row and theirsignals transit to a matrix of memory cells Σ via column bus BC to bestored (FIG. 2). A row is selected by means of signal RS.

Capacitor C2, of relatively low value to minimize read noise, presentson reset a noised level, called reference level, which is added to thesignal level (i.e the desired level) transferred from capacitor C1. Toattenuate the effect of this noise, Correlated Double Sampling (CDS) isgenerally performed, i.e. the reference levels of the pixels are firstsampled, and are then subtracted from the sampled signal levels aftertransfer from capacitor C1. This difference is generally made in anexternal memory (not represented).

Another type of pixel, called “shared pixel”, as described in US Patentapplication US200610256221, can be derived from that of FIG. 3 by addingthe elements in dashed lines, i.e. a second photodiode D1 b connected tonode A by a second transfer transistor TG′. The object of this structureis to reduce the space occupation of the pixels by sharing a readcircuit between several photodiodes. The level of each photodiode istransferred separately to bus BC in a phase as described in theforegoing. Capacitor C2 is thus reset at the beginning of each of thesetwo phases and cannot be used, as in a simple 4T pixel, to store theinformation of a previous image during acquisition of a current image.

FIG. 4 represents a column of pixels Px modified in order to improve themotion MTF.

To improve the motion MTF, two photodiodes are provided in each pixel.Integration of the charges associated with an image slice no longertakes place during a line time in a single photodiode, but during twohalf line times in two distinct photodiodes. This means that, bysuitably organizing the charge transfers of these photodiodes,integration of the charges can be synchronized with scrolling of theimage with greater fidelity, thereby limiting the motion effect (or“moving effect”) to a half line time (or to a pixel half-pitch).

Each pixel Px structurally looks like a shared pixel. It comprises twophotosensitive elements D1 and D1′ aligned in the direction of thecolumn. The photodiodes are preferably pinned photodiodes of the samedimensions. Photodiode D1 provided with integration capacitor C1 isconnected to storage node A (storage capacitor C2) via a transferelement TGM, for example a MOS transistor. In similar manner, secondphotodiode DI and its integration capacitor C1′ are connected to node Avia a second transfer transistor TGH.

Unlike a column composed of conventional shared pixels, each pixelcomprises a third transfer transistor TGB connecting storage capacitorC2 to photodiode D1′ of an adjacent pixel of the column. In FIG. 4,capacitor C2 of pixel Px of row n+1 (Px_(n+1)) is connected tophotodiode DI of the pixel of row n (Px_(n)), capacitor C2 of the pixelof row n is connected to photodiode D1′ of the pixel of row n−1, etc.

As in the circuit of FIG. 3, each pixel is provided with a followertransistor M2, a transistor RST connected to a supply voltage Vdd toreset capacitor C2, and a selection transistor, here referenced CS, totransfer a brightness level to a read bus L. This signal is routed to ananalog-to-digital converter (not represented) situated at one end ofread bus L.

In this configuration, read bus L is common to all the pixels of the rowand there is only one analog-to-digital converter per row. The gates oftransistors CS of the column are further connected to one and the samecontrol line, also referenced CS. This control line CS enables a wholecolumn of pixels to be selected for read.

Transistors TGH, TGM and TGB of each pixel P_(X) of FIG. 4 arecontrolled by a control circuit 15.

FIGS. 5 to 8 illustrate the operating steps of a column of pixelsaccording to FIG. 4. Successive image slices 18, 20 scroll along thecolumn of pixels in the direction of arrow 22 indicated in FIG. 5. Eachimage slice is divided into two halves to better illustrate thesynchronism between scrolling of the slice and the operating phases ofthe sensor. Operation is described in the following with relation to anytwo consecutive pixels Px_(n+1) and Px_(n).

FIG. 5 schematically represents a position of the image with respect tothe pixels during a first integration period T₁ corresponding to therolling time of an image half-slice in front of a photodiode.Photodiodes D1 and DI of pixel Px_(n) are exposed to an image slice 18and photodiodes D1 and D1′ of pixel Px_(n+1) are exposed to an imageslice 20. During this period T₁, integration capacitors C1 and C1′ ofeach pixel accumulate the charges corresponding to each slice. All thetransistors are off.

In FIG. 6, at the end of integration period T₁, transistors TGH and TGMare turned on and transfer the charges stored in capacitors C1 and C1′of the pixel to capacitor C2 of the pixel. Capacitor C2 of pixelPx_(n+1) receives the charges corresponding to the two halves of slice20 and capacitor C2 of pixel Px_(n) receives the charges correspondingto the two halves of slice 18. The other elements of the pixel,represented in dotted lines, are deactivated. Image slices 18 and 20have moved approximately a half of a pixel pitch during period T₁.

FIG. 7 schematically represents a position of the image during a secondintegration period T₂. Image slice 20 is astride the two adjacentpixels. The bottom part of slice 20 is captured by photodiode DI ofpixel Px_(n) whereas the top part of slice 20 is captured by photodiodeD1 of pixel Px_(n+1). This period therefore corresponds to passage of animage slice from a pixel to another. All the transistors are off. Duringperiod T₂, image slices 18 and 20 will also move a half pitch, therebymarking the end of a line time.

In FIG. 8, at the end of integration period T₂, transistors TGB and TGMare activated. In this configuration, the charges corresponding to thetwo halves of slice 20 integrated during period T₂ are again transferredto capacitor C2 of pixel Px_(n+1), where they are accumulated with thecharges previously stored in capacitor C2 for the same slice in the stepof FIG. 6.

After transistors TGB and TGM have been activated, capacitor C2 of pixelPx_(n+1) groups the brightness levels for two consecutive positions ofslice 20 and capacitor C2 of pixel Px_(n) groups the brightness levelsfor two consecutive positions of slice 18. These accumulated brightnesslevels can be extracted from the pixels by means of transistors M2 andCS at the end of period T2. In other words, each pixel contains doubleresolution data which simply has to be read at single frequency. Thisenables the motion MTF to be improved without increasing the temporalconstraints to read the data.

FIGS. 9A to 9H are timing diagrams summing up the global operation of acolumn of pixels according to FIG. 4. FIGS. 9A, 9B, 9D to 9G representthe control signals (or states) of transistors RST, CS, TGH, TGM, TGB.Signals 9C and 9H represent the activity of the analog-to-digitalconverter which processes the data after read via bus L.

Correlated Double Sampling is used to avoid reset noise. Activation ofselection transistor CS for read of the reference level REF isrepresented separately from activation for read of the (accumulatedsignal) brightness level SIG (FIG. 9B, 9G). Likewise, conversion oflevel REF is represented separately from that of level SIG (FIG. 9C,9H).

An exposure time T_(int) of a slice, generally corresponding to linetime T_(L), is defined between two successive activations of transfertransistor TGH or TGB. Integration periods T₁ and T₂ (T₁=T₂=T_(int)/2)take place during this time. The periodicity of the signals reflectsprocessing of the consecutive slices of the image.

The image slice is first of all scanned by photodiodes D1 and D1′ of onepixel during period T₁, as described in relation with FIG. 5. Beforetransferring the charges obtained in this way to capacitor C2, capacitorC2 is reset by activating transistor RST (FIG. 9A). Shortly after resetof capacitor C2, transistor CS is activated (FIG. 9B) to read and thenconvert reference level REF (FIG. 9C). As the conversion time T_(C) islonger than the duration of a pulse of signal CS, it is represented by abold line in FIG. 9C.

Once the reference level has been transferred to the converter,transistors TGH and TGM are activated to store the first brightnesslevels (FIG. 9D, FIG. 9E) which is marks the end of period T₁ and thebeginning of period T₂.

In period T₂, the image slice is scanned by photodiode D1 of the samepixel and photodiode D1′ of the next pixel. Transfer of the associatedbrightness levels is performed at the end of period T₂ by activation oftransistors TGM and TGB (FIG. 9E, FIG. 9F).

Once this charge transfer has been accomplished, the accumulatedbrightness levels SIG are read by activating transistor CS again (FIG.9G), and are then sampled by the analog-to-digital converter (FIG. 9H)before being stored in the matrix of memory cells Σ. A new exposurestarts with the next slice.

It can be observed that the central transfer transistor TGM is solicitedtwice as often as transistors TGB and TGH. Indeed, photodiode D1connected to transistor TGM scans the entire slice (i.e. twohalf-slices) whereas photodiodes D1′ (the one belonging to the pixel andthe one belonging to the adjacent pixel) only scan half a slice.

In FIGS. 9D to 9F, transistors TGH and TGM, or transistors TGB and TGM,are commanded simultaneously. They could also be activated one after theother.

During a line time, the pixel matrix has to be entirely read and summedin memory matrix 16. This read is performed column after column (rollingshutter mode) by reading the pixels of each column simultaneously. Theconverter of a line therefore performs as many conversions as there arecolumns in the pixel matrix during a line time T_(L). The timingdiagrams of a column adjacent to that of FIG. 4 are simply shifted by aconversion time T_(C).

Memory matrix 16 being managed in the same way as in the case of aconventional TDI-MOS sensor will not be described in detail in thepresent application.

Each image slice is thus scanned twice for a movement of one pixelpitch: a first time by photodiode D1 and a second time by photodiode D1′which is staggered a half pixel pitch with respect to photodiode D1.This particular oversampling enables the image motion MTF to besignificantly improved, going from 0.637 to about 0.9 (calculated at theNyquist frequency). Unlike prior art techniques, the pitch of a pixel isnot modified, neither is the spatial resolution of the image. The timeconstraints are therefore unchanged.

Actually, the temporal constraints are imposed by the read circuit andthe analog-to-digital converter. Indeed, for a matrix of m columns, mconversions (or 2m conversions in the case of CDS) have to be performedin a line time T_(L). The line time being predetermined according to thescanning rate of the image and the pitch, it is the speed of the readand conversion circuit that limits the number of columns of the matrix.

The invention claimed is:
 1. A time-delay-integration image sensorincluding a matrix of pixels organized in rows and columns, each pixelcomprising: a first photosensitive element; a storage node; a firsttransfer element connected between the first photosensitive element andthe storage node; a second photosensitive element; a second transferelement connected between the second photosensitive element and thestorage node; and a third transfer element connected between the storagenode and the second photosensitive element of an adjacent pixel of thecolumn, wherein the image sensor includes a control circuit configuredto simultaneously command the first and second transfer elements of eachpixel to an on state and the third transfer element of each pixel to anoff state, and in a distinct phase, to simultaneously command the firstand third transfer elements of each pixel to the on state and the secondtransfer element of each pixel to the off state, and the storage node isconnected to the first transfer element, the second transfer element andthe third transfer element in each pixel.
 2. The image sensor accordingto claim 1, further comprising: for each row of pixels, a read buscommon to the pixels of the row; and for each column of pixels, a readselection line common to the pixels of the column.
 3. The image sensoraccording to claim 2, further comprising: a matrix of memory cellsconnected to the read busses to store accumulated brightness levels ofseveral rows of pixels in a row of memory cells.
 4. The image sensoraccording to claim 1, further comprising: means for performingcorrelated double sampling of each pixel.
 5. The image sensor accordingto claim 1, wherein the first and second photosensitive elements arepinned diodes of the same dimensions.
 6. A method for management of atime-delay-integration image sensor including a matrix of pixelsorganized in rows and columns, the method comprising, for each pixel ofa column successive steps of: exposing first and second photosensitiveelements of the pixel during a first half period of an integrationperiod; transferring the brightness level of the first photosensitiveelement to a storage node of the pixel and transferring the brightnesslevel of the second photosensitive element to the storage node of thepixel in the first half period; exposing the first and secondphotosensitive elements of the pixel during a second half period of theintegration period; transferring the brightness level of the firstphotosensitive element to the storage node of the pixel and transferringthe brightness level of the second photosensitive element to a storagenode of an adjacent pixel of the column in the second half period; andreading the accumulated brightness levels in the storage nodes of thepixels.
 7. The method according to claim 6, further comprising: readinga reference level of the pixel before each read of the accumulatedbrightness levels of the pixel; and subtracting the reference level fromthe accumulated brightness levels.
 8. The method according to claim 6,wherein the pixels of one and the same column are commandedsimultaneously and the columns are addressed successively during one andthe same period.
 9. The method according to claim 6, wherein a number ofphotosensitive elements in the pixel in the first half period is equalto a number of photosensitive elements in the pixel in the second halfperiod.